Directory and paths handling

[guybedford]

From the discussion starting at #140 (comment).

Currently there is no way to allow paths to be relative to a different baseURL. Proposal is:

• Ensure directories.lib becomes the * -> lib/*.js default path.
• directories.base or something similar, and have directories.lib and directories.jspmPackages relative to this folder.

[guybedford]

Going with directories.baseURL after all, I think it will lead to the least confusion!

This is in the development branch now, will post a beta shortly.